Don't delete anything. If you do that, you'll lose valuable tracking info. You'll want to see, in our future reporting module, which listing sites sent you the best inquiries, which converted to bookings the most, etc.
You can certainly change the expiration to be expired if you want, but there's really no reason to unless you no longer want the person to be able to accept it. Just leave it as it, and it will expire on its own.
Our software takes care of automatically expiring quotes or preempting them (if someone else books the same dates). Once you respond to an inquiry or quote, you don't need to do anything else to get it to become inactive. Just let it go and the system will take care of it.
Again, DO NOT delete anything UNLESS you created something by mistake. It's best to leave the records around so you can find them later.